Abstract:

This thesis presents the results of a research project in which the enhancement of the CHP production by means of biofuel drying at an integrated pulp and paper mill has been explored. Instead of high temperature flue gas drying the use of low-temperature secondary heat as drying energy has been studied. The drying system considered in this thesis is classified as follows: Drying medium: Air Heat supply into the material: Convection Transport mechanism of the material inside the dryer: Conveyor Method to improve energy efficiency in the drying system: Multi-stage drying. In addition to secondary heat, the option of using steam as a drying energy has been included in the calculation models.

An optimization model for analyzing the integration of a multi-stage drying system into the CHP process has been developed. A simulation model based on the optimization model has been created to analyze the operation of the dryer under variable drying conditions. The simulation model can also be applied to the model-based control of the final fuel moisture content if there is a need to control it. Both models have been applied to a case study. Drying as a physical phenomenon has been studied experimentally in a small fixed bed dryer, and guidelines for dimensioning a continuous conveyor dryer in the case of multi-stage drying are presented. The energy efficiency of drying has also been analyzed using two evaluation methods: specific heat consumption and the irreversibility rate.

According to case study results, an optimally designed dryer uses only secondary heat, not steam, as a heat source, and earnings stem from decreased marginal fuel consumption, not increased power generation. Simulation calculations show that there are no economic grounds for control of the final fuel moisture content by adjusting heat inputs into the dryer. To control the final fuel moisture content the use of homogenous, and not just dry, fuel must have some positive influences on the operation of the boiler and/or power plant. Experimental tests show that critical moisture content is high for woody-based fuels and diffusion-based drying models must be used to determine drying times theoretically in a fixed bed. If the energy used in drying can be converted to mechanical work, the irreversibility rate is a more comprehensive method of comparing energy efficiency between different drying processes than specific heat consumption.Väitöstyössä esitetään tuloksia ja johtopäätöksiä tutkimusprojektista, jossa on tutkittu yhdistetyn sähkön ja lämmöntuotannon (CHP-tuotanto) energiatehokkuuden parantamista biopolttoaineen kuivauksen avulla integroidussa sellu- ja paperitehtaassa. Perinteisen savukaasukuivauksen sijasta tutkimuksessa on analysoitu matalalämpöisen sekundäärilämmön soveltuvuutta biopolttoaineen kuivaukseen. Laskentamallit on muodostettu kuivaussysteemille, jossa kuivauskaasuna toimii ilma, lämmönsiirto kuivauskaasusta materiaaliin tapahtuu konvektiolla, materiaalin siirto kuivurissa toteutetaan jatkuvatoimisella kuljettimella ja kuivurin energiatehokkuutta voidaan tarvittaessa parantaa ilman vaiheistuksen avulla (ns. monivaihekuivaus). Sekundäärilämmön lisäksi laskentamalleissa huomioidaan mahdollisuus käyttää vastapaine- ja/tai väliottohöyryä kuivausilman lämmityksessä.